[CMake] Make check-lldb work with LLDB_CODESIGN_IDENTITY=''

On Darwin, if a test machine isn't set up for code-signing (see
docs/code-signing.txt), running check-lldb should use the system
debugserver instead of the unsigned one built in-tree. This makes it
possible to run lldb's test suite without having code-signing set up,
which is really convenient.

Differential Revision: https://reviews.llvm.org/D42215

llvm-svn: 322803